Transformation and melting plants for metal charges are known, of the continuous charge type, which comprise an electric arc furnace suitable to collaborate with transport means of the linear conveyor type. The electric furnace provides at least a container, or hearth, and a covering roof. The electrodes are placed through suitable holes in the roof. A further hole, technically denominated the fourth hole, is normally provided in the roof for the extraction of the fumes.
The continuous conveyor means can be of the vibrating type in order to allow the charge to advance, and cooperate with the electric furnace on one side and with a scrap loading system on the other.
The last segment of the conveyor means normally consists of an independent conveying segment, hereafter called connection conveyor, associated with translation means, which may consist of a mobile slider or any other means of translation suitable for its operating positioning.
The translation means are suitable to create the desired connection between the continuous conveying means of the charge and the lateral introduction aperture of the metal material into the electric furnace. The front end of the connection conveyor is positioned flush with the inside wall of the furnace, for example in the continuous scrap loading step, or distanced from it, for example in the tapping step, when the furnace is inclined in order to tap the liquid metal.
When the end of the connection conveyor, associated with the translation means, reaches the inside wall of the furnace, the vibration imparted thereto determines the advance and the subsequent fall of the scrap inside the furnace. When, on the other hand, the connection conveyor is distanced from the furnace, the vibration is stopped, so that the scrap possibly present therein does not advance.
It is known that solutions of the continuous charge type use a loading system with baskets in order to carry out the first start-up load with the furnace switched off, in order to create on the bottom of the furnace a mass of metal material to be melted when the cycle starts. Normally the quantity of material introduced with the basket is the amount necessary to define the so-called “liquid foot”, that is, the quantity of liquid metal which is always kept inside the furnace even after tapping.
The use of the basket to carry out the first charge is determined by the fact that the switching on and start-up of the furnace require that the bottom of the hearth is covered by material that is sufficiently well distributed.
The use of a basket to carry out the first charge, although on the one hand it satisfies this need, on the other hand it makes the whole melting cycle more complex, in that it requires the use of specific equipment, such as the basket, the filling systems, the transport systems, a suitable storage zone etc., which are used for only a small fraction of the cycle, remaining idle for the rest of the melting cycle. Moreover, since the use of the basket means the scrap is loaded from above, the electrodes must first be completely extracted and moved to the side, the furnace roof must be opened and then the roof must be closed again and the electrodes re-introduced.
The cycle therefore becomes longer, due to the times needed for the first loading step, and the whole system becomes more onerous in terms of time and cost, it requires more equipment used, more operations to be carried out and coordination between a typical discontinuous loading step and the typical continuous loading steps.
The document U.S. Pat. No. 3,211,304 describes a furnace of the Martin-Siemens type in which a discontinuous loading system is provided which uses a longitudinally mobile container inside and outside the furnace.
The container, placed in a position outside the furnace, is loaded from above with the material, for example scrap, to be introduced into the furnace, and then progressively moved to the inside of the furnace.
Together with the container, in the direction it enters into the furnace, a barrier element moves, which, in the innermost position in the furnace, is also positioned inside the furnace.
When the container is retreated, on the contrary the barrier element remains stationary, so that the scrap is stopped by the barrier element which causes it to fall inside the furnace.
This solution therefore provides a discontinuous loading procedure using a longitudinally mobile container inside and outside the furnace, but it does not provide any type of continuous loading with vibrating conveyor.
The loading method provided by U.S. '304 can unload scrap only during the retreat of the container. Moreover, since the loading of the scrap is determined by a fixed barrier element, the unloading modes depend only on the distribution of the scrap on the container, and can in no way be controlled so as to define a desired distribution of the scrap inside the furnace.
With the device in U.S. '304 the scrap tends to accumulate in the part of the furnace near the entrance where the container is introduced, that is, in the fixed point where it stops and the barrier element, which causes the scrap to fall inside the furnace, is held stationary.
Purpose of the present invention is to solve the problems described above, eliminating the need to resort to a basket even for the first charge, and using for this operation the same equipment which is then used in the continuous load step when the furnace has been started.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.